The present participle used to form the continuous aspect

The present participle is used to form the continuous aspect (be + present participle) in continuous and perfect continuous tenses and infinitives:

Sorry, can you repeat that? I wasn't listening. (past continuous tense)
Your clothes are filthy! What have you been doing? (present perfect continuous tense)
You must be joking. (modal continuous)
If the kids weren't shouting all the time, perhaps I wouldn't feel so stressed. (past continuous subjunctive)
I happened to be passing your house, and I saw that the door was open. (continuous infinitive)
She is known to have been writing songs for years. (perfect continuous infinitive)